In an aesthetic crown or inlay restration, a material obtained by baking a pottery material called metal-bonded porcelain onto a metal core has heretofore been used. However, this dental prosthetic material has a problem in that the gum undergoes the discoloration due to elution of metal ions. Another problem is that a metal in the core shuts off light making it difficult to reproduce transparent feeling as that of a natural tooth.
On the other hand, a fully ceramic artificial crown (also called all ceramic artificial crown) of which the core, too, is formed of ceramics does not cause the gum to be discolored by the elution of metal ions, since no metal core is used. Besides, since a material having a transparent feeling close to a natural tooth is used as the ceramic core, it is made possible to realize a natural transparent feeling and to obtain a color feeling closer to a natural tooth due to the lamination of a special porcelain. So far, a metal has in many cases been used as a core of the artificial crown from the standpoint of strength. In recent years, however, technology has been developed to use, as a core material, a crystallized glass that exhibits a large strength (Japanese Unexamined Patent Publication (Kokai) No. 36316/1998, and a fully ceramic artificial crown has been much expected.
In preparing a fully ceramic artificial crown, in general, it is accepted practice to bake various porcelains onto the ceramic core, the porcelains having different color tones for each of the portions of the crown corresponding to constituent portions of a natural tooth in order to realize appearance close to a natural tooth. Concretely speaking with reference to FIG. 1 which is a sectional view of a fully ceramic artificial crown mounted on an abutment tooth F, it is a practice to bake, in the form of layers, a body porcelain A for reproducing an ivory color, an incisal porcelain B for reproducing an incisal color, a translucent porcelain C for producing a transparent feeling and, as required, a cervical porcelain D for reproducing a color of a neck of a tooth on the ceramic core E. These porcelains are generally obtained by blending a powdery ceramic component having an average particle diameter of from about 15 to about 100 μm with pigments that meet use of the porcelains.
A simple baking of the porcelains in an overlapped manner is not enough for reproducing a delicate color tone of a natural tooth or a pattern specific to an individual person. Therefore, it has been done to impart a color by using a porcelains called staining powder that contains a pigment in a relatively large amount, to smooth the surface and to impart transparency by glazing by baking a porcelain called glazing powder which does not almost contain pigment. Here, in order to realize a delicate color and a feeling of high surface quality, the ceramic component in the staining powder or in the glazing powder, usually, has an average particle diameter of from about 1 to about 15 μm, which is smaller than the average particle diameter of the body porcelain and so forth on described above.
And even in these staining powders, glazing powders and the above-mentioned various porcelains, porcelains as for the fully ceramic crown having coefficients of linear thermal expansion close to that of the crystallized glass used as the core material, and further having low firing temperatures and improved chemical resistances have been proposed. (Japanese Unexamined Patent Publication (Kokai) No. 139959/2000).
As the methods of producing a ceramic core used for the preparation of a fully ceramic artificial crown, there have been known a casting method in which the crystallization of the crystallized glass is performed by heat treatment after casting a molten crystallized glass into a mold, and a heated/pressurized molding method in which a crystallized glass is softened to a suitable degree without being melted and is poured into a mold to prepare a molded article.
According to the above heated/pressurized molding method, the crystallization of the crystallized glass is happened during the molding, making it possible to shorten the entire working time of molding inclusive of crystallization compared to the casting method, which is an advantage. According to this method, further, the ceramic material is slowly pored into the mold in a highly viscous state of about 102 to 106 poises without heating it nearly to its melting point, hence one can avoid involving bubbles and the reaction such as baking with the investment material (mold material) forming the mold during the molding, and one can obtain a ceramic artificial crown having stable properties (Japanese Unexamined Patent Publication (Kokai) No. 206782/1999).
Thus, the heated/pressurized molding method exhibits excellent characteristics as a method of producing ceramic cores requiring, however, an extended periods of time for molding the core since the highly viscous material is slowly poured.
In order to shorten the molding time, there can be contrived to increase the rate of molding the crystallized glass by increasing the load at the time of exerting pressure and elevating the temperature at the time of molding.
When the load is increased by using a weight, however, the weight of a large size must be used for the pressurized molding device. Even in other pressurized devices, it is necessary to employ a structure that withstands the pressure, arousing a problem in that the device tends to become bulky. Due to a large load, further, the surface of the investment material is rubbed by the crystallized glass that is poured. Depending upon the investment material that is used, therefore, the surface of the molded article is coarsened or the mold is broken. When the temperature is raised during the pressurized molding, further, the crystallization proceeds faster, and it becomes difficult to control the crystallization and the quality of the obtained core decreases.
According to the heated/pressurized molding method as described above, a new problem arises when it is attempted to shorten the molding time by increasing the load at the time of exerting pressurize or by elevating the molding temperature. It has, therefore, been urged to provide a method of shortening the molding time without accompanied by the above-mentioned problem.
The dental porcelain favorably used as the ceramic core surely exhibits excellent properties. When it is really used, however, the color of the core ceramics tends to be seen through due probably to that light is not suitably scattered inside thereof. To obtain a color tone comparable to that of a natural tooth, therefore, it is necessary for the dental porcelain to impart a suitable degree of transparency which inhibits the underlying color from being seen through.